Exterior roof insulation system with metal roof
I have a circa 1850 timber frame house near Syracuse NY (zone 5a). We get a lot of snow! This house needs everything – siding, roof, windows so it is ideal for installing exterior insulation. In addition to renovation of all of the exterior I will be constructing an addition. I have a good feel for what I want to do on the addition. The existing roofs are 7/12 and 5/12 slopes. There is one small valley but otherwise are simple gables. Rafters are small and widely spaced. Decking is wide rough-sawn boards in good shape but as is typical for this era house, there are gaps between boards, splits, etc. I have a few questions on the renovations:
1. Is there a chance I can install peel and stick membrane directly to board roof sheathing or should I plan on adding OSB on top of boards? Roofs have lots of slope and membrane will be covered fairly quickly. It seems that one of the lesser grades of peel and stick is ok in this method.
2. I plan to use exposed fastener metal roofing. Per suggested detail I will install 2x4s attached through foam (I plan to use 2 layers 2″ XPS) to create vent space (aligned with rafters). I was going to add 2×4 purlins on top of first layer of 2x4s to attach roof to. I can find no details or photos of roof installed this way. Has this been done successfully?
3. Can I just lap the joints in foam, and tape the joints under the roofing? Felt or some other membrane on top of foam seems like a waste of resources, as taped foam will shed moisture and peel and stick is the last line of defense.
4. I plan to open up ceilings to expose the timber frame where I can. I’ll have to somehow improve the rafters but it may be difficult to add enough depth to put R-30 below the sheathing to get to R-49. It doesn’t seem to matter if I underachieve here, correct? It will be so much better than what it’s replacing (anyone else have vermiculite insulation in their attic?
Enough for now. Thanks in advance for the input!
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